As far as I understand it, noise is just the deviation from or distortion of a pattern/signal/function/etc. Where noise in an image would be pixels of random color, noise in sound would be like static or "white noise". Just seemingly random nonsense. Adding noise to a function that you might use in Black Ink will just give it a bit of randomness. There are different types (that you can see by calling up the node). This is like the opposite of the "Smooth" node, which will level out its input function.